Method of producing a moisture permeable sheet material

ABSTRACT

A method of imparting moisture permeability to a sheet material having a porous base and a non-porous surface layer is provided comprising impregnating the pores within the porous base with an aqueous medium, subjecting the impregnated sheet material to electrical discharge, thereby forming a plurality of perforations in said non-porous surface layer, and recovering the sheet material exhibiting enhanced moisture permeability.

United States Patent Harada [151 3,665,157 451 May 23, 1972 54,] METHODOF PRODUCING A MOISTURE PERMEABLE SHEET MATERIAL [72] Inventor:Kazuto-l-larada, Kurashiki, Japan [73] Assignee: Kuraray Co., Ltd.,Kurashiki Japan [22] Filed: Feb. 19, 1971 [21] App]. No.: 1 17,143

[30] Foreign Application Priority Data Feb. 17, 1970 Japan ..45/l748lJune 12, 1970 Japan ..45/51403 [52] US. Cl ...2l9/384,83/l6, 219/284,346/74 [51] Int. Cl. ..H05b 7/18 [58] Field of Search ..2l9/383, 384,234, 284; 83/16, 83/169, 170; 346/74 [56] References Cited UNITED STATESPATENTS 853,755 5/1907 Belin ..219/383X 2,107,931 2/1938 2,481,048 9/1949 2,550,366 4/1951 3,164,716 1/1965 Schenker et a]. ..219/3833,183,518 5/1965 Henry et a1. ..346/76 3,424,895 1/1969 Olson 2l9/ 384Primary Examiner-Volodymyr Mayewsky Attorney-William Kaufman and BarryKramer [57] ABSTRACT A method of imparting moisture permeability to asheet material having a porous base and a non-porous surface layer 19Claims, NO Drawings METHOD OF PRODUCING A MOISTURE PERMEABLE SHEETMATERIAL This invention relates to a method of producing sheet materialsexhibiting excellent moisture permeability. More particularly, thisinvention relates to an improved method for electrically introducingperforations in a non-porous surface layer of sheet material to provideenhanced moisture permeability thereto.

Leather substitutes have heretofore been developed for use in themanufacture of shoes, boots, trunks, bags and the like. Generally, theseleather substitutes comprise sheet materials .produced by forming aporous base material and covering said base material with a non-porouslayer which can be colored or uncolored depending upon the desiredeffect to be obtained. The resulting sheet material is then embossed toimpart a leather appearance thereto. When, however, the sheet materialsare finished with sufficient thickness to provide a good appearance, ithas inevitably been found that the moisture permeability of the sheetmaterial is substantially impaired.

Methods have heretofore been developed for improving the moisturepermeability of vinyl sheets and the like wherein electrical dischargessuch as corona discharges or pulse discharges have been employed toimpart minute perforations or pores in the non-porous surface layer.Although this electrical discharge process can be employed toimpartperforations to the non-porous surface layer of sheet materialscomprising a porous base material and a non-porous surface layer, it hasbeen found that the presence of the porous base material between thedischarge electrodes necessitates the use of higher voltage to obtainthe necessary degree of perforation. When using higher voltage, however,it is extremely difficult to obtain uniform perforation of the surfacelayer; moreover, the resulting perforations are larger than desired.

It is therefore an object of this invention to provide a method forimparting moisture permeability to sheet materials which overcomes theabove-noted deficiencies.

It is another object of the present invention to provide a method forproducing sheet materials exhibiting uniform and minute perforations aswell as excellent moisture permeability by an electrical dischargeprocess.

These as well as other objects are accomplished by the present inventionwhich provides a method of imparting moisturepermeability to a sheetmaterial having a porous base and non-porous surface layer comprisingimpregnating the pores within the porous base with an aqueous medium,subjecting the impregnated sheet material to electrical discharge,thereby forming a plurality of perforations in said non-porous surfacelayer, and recovering the sheet material exhibiting enhanced moisturepermeability. The aqueous medium'employed in the present invention canbe water or an aqueous electrolytic solution. if water is employed asthe aqueous medium, the sheet material can be recovered by drying. if,however, an aqueous electrolytic solution is employed, the

treated sheet material can be first washed with water and then dried. ineither case, sheet material exhibiting numerous minute pores therein andexhibiting excellent moisture permeability is obtained.

The porous base material of the sheet material of the present inventionexhibits a high breakdown voltage. When impregnated with an aqueousmedium, it exhibits a higher electrical conductivity than when it is notso impregnated. Thus, the electrical discharge process is facilitatedenabling a great number of uniform and minute perforations to beobtained in the non-porous surface layer of the sheet material.Additionally, because of the higher electrical conductivity obtained inaccordance with the present invention, the initial discharge voltage ofthe electrical discharge can be decreased thereby enabling finer andmore uniform perforations to be obtained.

It .is considered preferable to employ an aqueous electrolytic solutionin accordance with the present invention 'since the electricalconductivity of said solution is far higher than that obtained by simplyimpregnating the porous base layer with water. However, it has beenfound quite convenient to employ plain water. Even ordinary tap watercan be employed successfully to impart excellent moisture penneabilityto the sheet material. In the absence of impregnation with an aqueousmedium in accordance with the present invention, electrical discharge ofsheet material of the type disclosed herein. fails to impart numerousand uniform perforations to the non-porous surface layer.

The sheet material of the present invention may be impregnated by anyconvenient method such as immersion, spraying, padding or the like withan aqueous medium. If desired, the sheet material can be impregnated bya combination of immersion and pressing. If desired the impregnationtreatment can be affectively expedited by employing a higher temperaturefor the aqueous medium and/or employing a penetrating agent such as asurfactant.

The aqueous medium employed from impregnation of the base material canbe ordinary tap water or water to which an electrolytic material hasbeen added. Suitable electrolytic materials are acids such ashydrochloric acid, sulfuric acid, formic acid, trichloroacetic acid,acetic acid, and other acids; bases such as sodium hydroxide, potassiumhydroxide, ammonium hydroxide and the like, as well as salts such assodium chloride, sodium sulfate, sodium carbonate, calcium chloride andthe like. Preferably the salts which are employed exert no adversehydrolytic effects on the polymers employed in the sheet material. Theconcentration of the electrolyte in the solution depends upon theelectrolytic material employed. Generally, however, it is considereddesirable that the concentration of the electrolytic material be atleast about 1 percent by weight and can range up to the solubility limitof the particular electrolyte at the temperature at which the aqueousmedium is employed, however, the concentration should be below thatwhich would cause hydrolytic attack on the polymers employed in thesheet material. Since dilute electrolytic solutions are effective in thepresent invention and most economical, they are preferred. it isconsidered preferable that the prous base material be impregnated withthe aqueous medium to the extent of at least about percent by weight.

In operation, the porous base layer of the sheet material is impregnatedwith an aqueous medium and placed on the grounded electrode of anelectrode pair. The other electrode is employed to effect an electricaldischarge through the nonporous surface of the sheet material. Thislatter electrode, i.e., the non-grounded electrode, can be of anyconvenient shape but is desirably in the form of a needle. The waveformof the voltage applied to the electrode pair can be selected as desiredto give rise to a spark discharge. Preferably, the waveform is arectangular or semi-rectangular waveform giving rise to a pulse sparkdischarge. The voltage, frequency and duration of discharge will varydepending upon the material being treated. By suitably selecting thevoltage, frequency and pulse width with respect to a given material, thenumber, diameter and depth of the perforations imparted to thenon-porous surface layer and the appearance thereof can be altered.

The porous base layer of the sheet material of the present inventioncomprises fibrous materials such as paper, woven fabric, non-wovencloth, knitted fabric and other similar fibrous materials. Also, theporous base layer can comprise said fibrous materials impregnated with apolymer to provide a porous and bulky base material. Further, the porousbase material can consist of two layers namely a porous polymer layerobtained by impregnation of a fibrous material layer and a secondfibrous material layer. Both of said layers can be additionallyimpregnated with a polymer to provide a porous and bulky structure. Theporous base material exhibits a substantial degree of porosity and thusa high breakdown voltage. By

filling these pores with an aqueous medium in accordance with thepresent invention, the effective discharge distance to the non-poroussurface layer is decreased thereby facilitating the electricaldischarge. Moreover, the discharge is further facilitated by theenhanced electrical conduction afforded by the presence of theimpregnated aqueous medium.

The non-porous surface layer to which improved moisture permeability isimparted in accordance with the electrical discharge process of thepresent invention adheres to the surface of the porous base material.This surface layer comprises a non-porous polymeric film generallyhaving a thickness of from about 0.001 to about 0.05 centimeters. Thenon-porous surface layer is applied to the surface of the porous basematerial by spraying or coating a solution or dispersion of the polymerthereon or by bonding a preformed polymeric film thereto. The non-poroussurface layer can also be prepared by partial dissolution and drying orpartial melting and cooling of the surface of the porous polymer layer.The non-porous surface layer imparts an excellent surface appearance tothe sheet material and also imparts excellent anti-scratch resistancethereto. Additionally, the resultant sheet material exhibits a good handor feel due to interaction between the porous base material andnon-porous surface layer.

The polymers of the porous base layer and the non-porous surface layermay be polyurethane elastomers, polyamides, polyesters, polyamino acidresins, polymethacrylates, polyvinyl chlorides, polyvinyl acetates,polyisoprenes, copolymers of butadiene and acrylonitrile. They may beused alone or in admixture with each other.

The electrical discharge process of the present invention impartsnumerous uniform fine pores having diameters ranging from about 1 toabout 100 microns to the non-porous surface layer of the sheet material.These fine pores are directly connected to the porous base materialthereby improving the moisture permeability of the sheet materialwithout deteriorating the appearance of the sheet material or itsphysical properties.

The sheet material of the present invention can be employed for manypurposes wherein the excellent physical properties, appearance, touch,moisture permeability and the like can be used to advantage. Forexample, the sheet material of the present invention can be employed inthe manufacture of shoes, boots, clothes, gloves, headgear and likewearing apparel, trunks, bags, furniture covering, interior decorationand the like. i

The following examples further define, describe and compare methods ofpreparing the improved sheet material of the present invention. Partsand percentages are by weight unless otherwise indicated.

EXAMPLE 1 An unwoven cloth of nylon fibers was impregnated with apolyurethane elastomer solution. The same polyurethane elastomersolution was further spread upon the impregnated cloth and the resultingcloth was then coagulated into a porous structure in a coagulation bath.After the coagulating liquid was 'washed out and the cloth dried, thesurface of the resultant porous sheet base material was coated with afinishing solution consisting of the polyurethane elastomer to athickness of microns and was then pressed by passage through rollersheated at 160 C. By applying a coating of the finishing solution andpartially melting the porous polyurethane elastomer layer due to thepressing operation a operation, non-porous surface layer was formedabout microns thick and was firmly bonded to the porous base. Theresulting sheet material exhibited an improved appearance and enhancedphysical properties; however, its moisture permeability dropped from1,560 gm./m /day to 390 gm./m /day.

The above sheet material was then dipped in hot water at 80 C. andpressed and the dipping operation and pressing were repeated until theporous base material was thoroughly impregnated with water. Theimpregnated sheet material was placed on a grounded electrode plate.Rectangular wave pulses 2,100 volts, 1,000 Hz, 5 microseconds wide wereapplied to an electrode situated above the sheet material causingpulsating electrical discharges to be impressed upon the nonporous layerof the sheet material resulting in the formation of pores in saidsurface layer having an average diameter of about 35 microns and a poredensity of about 104 pores per square centimeter. After the electricaldischarge treatment, the sheet material was dried. The moisturepermeability of the resultant product was raised to 930 gmJm ldaywithout deteriorating the appearance or physical properties of theproduct.

For purposes of comparison, an identical sheet material which was notimpregnated with water as described above failed to give rise toelectrical discharge and perforation of the non-porous surface layerunder the identical conditions described above.

EXAMPLE 2 A porous base material comprising a first layer of an unwovencloth of nylon fibers impregnated with a polyurethane elastomer and asecond layer bonded to said first layer comprising a porous polyurethaneelastomer structure, was coated with a non-porous film consisting ofpolyurethane elastomer having a thickness of 14 microns forming a sheetmaterial exhibiting excellent appearance, physical properties and touch.

The resulting sheet material was subjected to repeated pressing in watercontaining 1 weight percent of a nonionic surfactant at 50 C. forthorough impregnation. The thus treated material was placed on agrounded electrode plate. Electrical pulses 1,500 volts, 1,000 Hz, 50microseconds wide were applied to an electrode situated above the sheetmaterial thereby subjecting the surface layer thereof to pulsedischarges giving rise to the formation of perforations in said surfacelayer. Thereafter the sheet material was washed and dried. Numerousuniform pores having an average diameter of about 25 microns were formedin the surface layer at a pore density of about 842 pores per squarecentimeter. The moisture permeability of the sheet material wasincreased from 410 gm./m /day prior to treatment to 1,850 gm./m /dayafter treatment without in any way impairing the appearance, touch, orother physical properties of the sheet material.

For purposes of comparison, the identical sheet material was notimpregnated with water and was found not to undergo electrical dischargeor the generation of perforations in the surface layers thereof underthe identical conditions.

EXAMPLE 3 A sheet of unwoven cloth of nylon fibers was impregnated witha polyurethane elastomer solution and was then overcoated with thepolyurethane elastomer solution and coagulated in a coagulation bath toform a porous structure. After removing the coagulating liquid anddrying, a porous base material was obtained. The resultant base materialwas coated with a finishing solution of polyurethane elastomer to athickness of 5 microns and the resultant sheet material was pressed bybeing passed through heated rolls maintained at C. Through applicationof the finishing solution and partial melting of the surface of theporous polyurethane elastomer layer, a beautiful non-porous surfacelayer about 20 microns thick was obtained exhibiting improved appearanceand physical properties. The moisture permeability of the producthowever decreased from 1560 gm./m /day to 390 gm./m /day.

The resulting sheet material was immersed in a 5 percent by weightaqueous solution of sodium chloride at 70 C. and was repeatedly pressedtherein to thoroughly impregnate the porous base layer of the sheetmaterial with said solution. Thereafter the impregnated sheet materialwas placed on a grounded electrode plate and rectangular wave pulses2,000 volts, 1,000 Hz, 10 microseconds wide were impressed upon anelectrode situated above the sheet material giving rise to an electricaldischarge through the surface layer resulting in formation of numerousperforations in said surface layer. Thereafter the sheet material waswashed with water and dried. The resulting product exhibited numerousuniform perforations in the surface layer thereof having an averagediameter of about 10 microns, and an average pore density of about 1,420perforations per square centimeter. The moisture permeability of theresulting product was found to be 1,190 gm./m /day as compared to 390gm./m /day for the untreated material. The appearance and other physicalproperties of the sheet material were not impaired by the electricaldischarge treatment.

For purposes of comparison, sheet material of the type described abovewas impregnated with water and subjected to electrical discharge underthe same conditions as described hereinabove. The moisture permeabilityof the resulting product was increased to 730 gm./m /day.

EXAMPLE 4 A porous base material comprising a first layer of unwovencloth of nylon fibers impregnated with a polyurethane elastomer and asecond layer bonded thereto comprising a porous polyurethane elastomerstructure was surface coated with a non-porous film of polyurethaneelastomer 4 microns thick to produce a sheet material exhibiting anexcellent appearance, physical properties and touch.

This sheet material was thoroughly immersed in a 5 percent weightaqueous solution of sodium sulfate at 70 C. with repeated pressing.

The impregnated sheet material was placed on a grounded electrode plateand rectangular wave pulses, 2,000 volts, 1,000 Hz, 7 microseconds widewere applied to an electrode situated above the sheet material givingrise to electrical discharges through the surface layer of the sheetmaterial. The resultant material was washed and dried and found toexhibit numerous minute perforations in the surface layer thereof havingan average diameter of 34 microns and an average pore density of about312 pores per square centimeter. The moisture permeability of theresultant product was increased from 410 gm./m /day before treatment to1,390 gmJm lday after treatment without impairing the appearance, touchand other physical properties of the sheet material.

What is claimed is:

1. Method of imparting moisture permeability to a sheet material havinga porous base exhibiting a high breakdown voltage and a non-poroussurface layer comprising impregnating the pores within the porous basewith an aqueous medium to increase the electrical conductivity of saidporous base, placing the impregnated sheet material between an electrodepair, subjecting the impregnated sheet material to electrical dischargebetween said electrode pair, thereby forming a plurality of perforationsin said non-porous surface layer, and recovering the sheet materialexhibiting enhanced moisture permeability.

2. Method as defined in claim 1 wherein the aqueous medium is water.

Method as defined in claim 2 wherein said water additionally contains asurfactant.

4. Method as defined in claim 1 wherein the aqueous medium is an aqueouselectrolytic solution.

5. Method as defined in claim 4 wherein the electrolytic solutioncontains an electrolyte in the amount of at least about 1 weight percentup to about the solubility limit of the electrolyte in water at thetemperature of impregnation.

6. Method as defined in claim 1 wherein the sheet material isimpregnated by immersion in said aqueous medium and pressing of saidsheet material thereby thoroughly impregnating the porous base of suchsheet material with said aqueous medium.

7. Method as defined in claim 6 wherein the sheet material is immersedin the aqueous medium and pressed until the porous base has beenimpregnated with at least about weight percent of the aqueous medium.

8. Method as defined in claim 1 wherein a rectangular wave formelectrical pulse is applied to one of the electrodes of said electrodepair.

9. Method as defined in claim 1 wherein the electrical discharge is apulse spark discharge.

10. Method as defined in claim 1 wherein the porous base material is afibrous material impre nated with a polymer.

11. Method as defined in claim 0 wherein the polymer is a polyurethaneelastomer.

12. Method as defined in claim 1 wherein the non-porous surface layer isof a thickness ranging from about 0.001 to about 0.05 centimeters.

13. Method as defined in claim 1 wherein the perforations formed in thenon-porous surface layer after electrical discharge range in diameterfrom about 1 to about microns.

14. Method as defined in claim 2 wherein the treated sheet is recoveredby drying.

15. Method as defined in claim 4 wherein the treated sheet material isrecovered by washing the subsequent drying.

16. Method as defined in claim 1 wherein the porous base materialcomprises a first layer of a fibrous material impregnated with a polymerand a second layer of fibrous material.

17. Method as defined in claim 16 wherein the polymer is a polyurethaneelastomer.

18. Method as defined in claim 16 wherein said first and second layersare additionally impregnated with a polymer.

19. Method as defined in claim 18 wherein the polymer is a polyurethaneelastomer.

2. Method as defined in claim 1 wherein the aqueous medium is water.Method as defined in claim 2 wherein said water additionally contains asurfactant.
 4. Method as defined in claim 1 wherein the aqueous mediumis an aqueous electrolytic solution.
 5. Method as defined in claim 4wherein the electrolytic solution contains an electrolyte in the amountof at least about 1 weight percent up to about the solubility limit ofthe electrolyte in water at the temperature of impregnation.
 6. Methodas defined in claim 1 wherein the sheet material is impregnated byimmersion in said aqueous medium and pressing of said sheet materialthereby thoroughly impregnating the porous base of such sheet materialwith said aqueous medium.
 7. Method as defined in claim 6 wherein thesheet material is immersed in the aqueous medium and pressed until theporous base has been impregnated with at least about 80 weight percentof the aqueous medium.
 8. Method as defined in claim 1 wherein arectangular wave form electrical pulse is applied to one of theelectrodes of said electrode pair.
 9. Method as defined in claim 1wherein the electrical discharge is a pulse spark discharge.
 10. Methodas defined in claim 1 wherein the porous base material is a fibrousmaterial impregnated with a polymer.
 11. Method as defined in claim 10wherein the polymer is a polyurethane elastomer.
 12. Method as definedin claim 1 wherein the non-porous surface layer is of a thicknessranging from about 0.001 to about 0.05 centimeters.
 13. Method asdefined in claim 1 wherein the perforations formed in the non-poroussurface layer after electrical discharge range in diameter from about 1to about 100 microns.
 14. Method as defined in claim 2 wherein thetreated sheet is recovered by drying.
 15. Method as defined in claim 4wherein the treated sheet material is recovered by washing thesubsequent drying.
 16. Method as defined in claim 1 wherein the porousbase material comprises a first layer of a fibrous material impregnatedwith a polymer and a second layer of fibrous material.
 17. Method asdefined in claim 16 wherein the polymer is a polyurethane elastomer. 18.Method as defined in claim 16 wherein said first and second layers areadditionally impregnated with a polymer.
 19. Method as defined in claim18 wherein the polymer is a polyurethane elastomer.